1. Field of the Invention
The invention relates to syringes by which medicine may be apportioned in preset doses from an ampoule, the syringes comprising a dose setting mechanism which is through unidirectional coupling coupled to a piston rod drive mechanism which advances the piston rod into the ampoule so that the dose setting mechanism may be moved in one direction to set a dose without transmission of the movement of the piston rod, and in another direction to inject the set dose, by which the movement is transmitted to the piston rod to advance it a distance causing an amount of medicine corresponding to the set dose to be pressed out of the ampoule.
The medicine is pressed out through a needle which is mounted on the syringe communicating with the inner space of the ampoule. To inject a dose of medicine a dose is set by operation of the dose setting mechanism, the needle is inserted through the skin of the user or patient, and the set dose is injected by operating the dose setting mechanism in the. opposite direction of the direction used by the setting of the dose.
2. Description of the Related Art
EP 37 696 describes a medicine dispensing device wherein a unidirectional coupling between a first drive member and a ratchet-toothed second drive member driving the piston transmits only movement of the first drive member in an piston advancing direction to the second drive member.
WO 93/07922 describes a syringe by which a dose is set by rotating a dose setting knob in one direction. The dose setting knob is connected with a part of an unidirectional coupling through which the dose setting knob is coupled to a piston drive comprising a nut which is so journaled in the housing that it may be rotated but not longitudinally displaced in this housing. An inner thread in the nut engages an outer thread on a piston rod which rod may be displaced longitudinally in the housing but not rotated. The unidirectional coupling between the dose setting knob and the piston drive nut is so directed that only piston advancing rotation is transmitted to the piston drive nut.
However, before the dose is set and made ready for injection it must be ensured that no air is injected. To get rid of possible air in the needle or in the ampoule a small dose of the medicine, e.g. 10 or 20 .mu.l, is set and an injection operation is made without insertion of the needle into the skin. By this so called air shot air is driven out through the needle. At the point of the needle it can easily be seen if all air has been expelled and only liquid is leaving the needle. If a fine liquid jet is not seen at the end of the air shot a new small dose must be set and a new air shot be made until such a jet is seen.
To avoid to have to perform repetitive settings of a small dose and subsequent air shot the user may feel an impulse to set a somewhat larger dose to make sure that only one air shot has to be made. Thereby unnecessary amounts of expensive medicine may be wasted. Also in many syringes of the kind described no undo mechanism exists so that a dose once set cannot just be annulled, or at least this will demand special precautions. Therefore if a dose is set and the user thereafter remember that he forgot to make an air shot the dose must be wasted or annulling precautions must if possible be taken. Further, during the air shot the syringe must be operated with the needle pointing upward. As most syringes are designed to be operated with the needle pointing mainly downward, operation with the needle pointing upward may be rather awkward.
It is an object of the invention to provide a syringe by which these disadvantages may be avoided.